1. Field of the Invention
This invention relates generally to an apparatus and method for acquiring an at least one dimensional image of an object and in particular to an apparatus and method for acquiring a series of at least one dimensional images of an object at high rates using a charge coupled device (CCD) array, whereby each at least one dimensional images are captured in a parallel fashion without requiring any transverse scanning.
2. Description of Related Art
There are many industrial, medical, and other applications where one or two dimensional images of an object are required. In addition, these applications often require both high spatial resolution and high longitudinal resolution (less than 10 micrometers), measurements of distances, thicknesses, and optical properties of the object. The applications can include measurements of biological tissue layers, semiconductors and other applications involving multiple thin layers of material, as well as in the non-destructive testing of small structures such as integrated optical circuits, optical connectors, optical couplers, semiconductor lasers and semiconductor optical amplifiers. Such applications also include various medical applications including laser microsurgery, microscopy and diagnostic instrumentation.
Existing techniques for acquiring one or two dimensional images include scanning laser or confocal microscopes and scanning laser ophthalmoscopes (SLO), provide highly spatially resolved images, for example being able to generate real time video images of the eye with a lateral resolution of a few micrometers. However, the depth resolution of SLOs quickly degrade with decreasing numerical aperture. For example, SLO measurements of the retina through the pupil aperture restrict the depth resolution to roughly 200 microns. SLOs are also expensive, costing in the range of a quarter million dollars.
Optical triangulation offers fairly high resolution, but requires parallel boundaries. Such devices also have relatively poor signal-to-noise ratios and have degraded resolution at greater depths, where numerical aperture is restricted.
Existing techniques for performing such measurements include optical coherence domain reflectometers (OCDR), optical time domain reflectometry (OTDR), ultrasound, scanning laser microscopes, scanning confocal microscopes, scanning laser ophthalmoscopes and optical triangulation. Existing OCDR systems do not normally have the rapid data acquisition rate required for the measurement of biological or other samples having the potential for dynamic movement; while OTDR systems are very expensive and have only limited resolution and dynamic range.
Ultrasound, which is perhaps the most commonly used technique, is disadvantageous for applications such as taking measurements on the eye in that, in order to achieve the required acoustic impedance matches, and to thus avoid beam losses and distortion, contact is generally required between the ultrasonic head or probe and the product or patient being scanned. While such contact is not a problem when scans are being performed on, for example, a patient's chest, such probes can cause severe discomfort to a patient when used for taking eye measurements such as those used for measuring intraocular distances for computing the power of lens implants.
The relatively long wavelengths employed in ultrasound also limit spatial resolution. Further, ultrasound depends on varying ultrasound reflection and absorption characteristics to differentiate and permit recording or display of tissue, or other boundaries of interest. Therefore, when the acoustic characteristics of adjacent layers to be measured are not significantly different, ultrasound may have difficulty in recognizing such boundaries.
A need, therefore, exists for an improved method and apparatus for performing high resolution measurements and in particular for optically performing such measurements, which improved technique does not require contact with the body being measured, which maintains substantially constant high resolution over a scanning depth of interest, regardless of available apertures size and which is relatively compact and inexpensive to manufacture. Such a system should also be capable of providing differentiation between sample layers and be able to provide identification of layer material or of selected..properties thereof. Such a system should also be able to provide one, two and three-dimensional images of a scanned body and should be rapid enough for use in biological and other applications where the sample being measured changes over relatively short time intervals. Finally, it would be desirable if such technique could also provide information concerning the birefringence property and spectral properties of the sample.
U.S. patent application Ser. No. 08/033,194 relates to optical measuring systems which can perform high resolution measurements and provide the above advantages. In particular, these systems can perform such measurements without contacting the object or body being measured. The systems maintain substantially constant high resolution and are relatively compact and inexpensive to manufacture. Such systems are also capable of providing differentiation between sample layers, identification of layer material or of selected properties thereof. The systems also provide measurements at rapid enough rates for use in biological and other applications where the sample being measured changes over relatively short time intervals. In fact, they can even provide information concerning the birefringence property and spectral properties of the sample.
FIG. 1 shows an object 28 being scanned by an optical beam. In particular, a first beam 101 is incident on a scanning mirror 71 which reflects beam 101 as beam 103 which is incident on object 28 at region A. Mirror 71 rotates about pivot P in a controlled manner by some type of rotating driver (not shown) which causes mirror 71 to move to a new position represented by reference numeral 71'. As mirror 71 moves to its new .position, beam 103 scans across object 28 in the transverse direction to region B. As beam 103 passes across object 28, object 28 scatters the radiation back towards some detection unit (not shown) which detects that scattered radiation coherently using radiation from a reference arm (not shown).
The detection unit for capturing a one or two dimensional image of object 28 involves capturing a series of values corresponding to a series of intensity measurements. This series of intensity measurements in turn correspond to a series of regions of object 28 transversely scanned by beam 103 across object 28. Hence, this approach to acquiring measurements across a transverse direction of an object is essentially a serial image capturing process. Consequently, the rate of image capture by such a serial system may be limited by the rate at which individual measurements are acquired and hence at the rate beam 103 is scanned across object 28. This holds regardless of whether or not beam 103 is scanned using a mechanical scanner such as mirror 71 or some other non-mechanical scanning mechanism such as acousto-optic devices. In addition, if a second scanning means such as a second mirror is added to enable scanning in a second transverse direction, the rate of acquisition of resulting two dimensional images is even further reduced.
In some applications, the power of radiation incident on a unit area is fixed, e.g., optical measurements of the human eye.illuminating a human retina. In such cases, simultaneous or parallel illumination and acquisition of information represents a more efficient approach to performing optical measurements than any transverse scanning approach.
Hence it is further desirable to perform a longitudinal scan through an object while acquiring one or two dimensional slices in a completely parallel fashion as the longitudinal scan proceeds. It is desirable to acquire these one or two dimensional images without the use of transverse scanning mirrors which can slow down the rate of image capture and introduce noise which reduce system sensitivity and resolution.